Laser resonator for improving narrow band emission of an excimer laser

ABSTRACT

An apparatus and method are provided for bandwidth narrowing of an excimer laser to Δλ≈6 pm or less with high spectral purity and minimized output power loss. Output stability with respect to pulse energy, beam pointing, beam size and beam output location is also provided. The excimer laser includes an active laser medium for generating a spectral beam at an original wavelength, means for selecting and narrowing the broadband output spectrum of the excimer laser, a resonator having at least one highly reflecting surface, and an output coupler. Means for adapting a divergence of the resonating band within the resonator is further included in the apparatus of the invention. The divergence adapting causes the spectral purity to improve by between 20% and 50% and the output power to reduce by less than 10%. A method according to the invention includes selecting and aligning the divergence adapting means.

PRIORITY

[0001] This application is a 37 C.F.R. 1.53(b) continuation applicationof U.S. patent application Ser. No. 09/923,632, filed Aug. 6, 2001,which is a continuation of U.S. patent application Se. No. 09/130,277,filed Aug. 6, 1998.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The invention relates to a resonator designed fornarrow-linewidth emission, and particularly to a resonator for anexcimer laser having optical components for improving spectral purity,reducing spectral bandwidth and optimizing output power for emitting ahigh resolution photolithographic beam.

[0004] 2. Discussion of the Related Art

[0005] To increase the capacities and operation speeds of integratedcircuits, manufacturers are inclined to design smaller internalstructures for devices and other components of these chips. Thereduction in size of a structure produced on a silicon wafer is limitedby the ability to optically resolve the structure. This resolutionability depends directly upon the photolithographical source radiationand optics used.

[0006] Excimer lasers emitting pulsed UV-radiation are becomingincreasingly important instruments in specialized material processing.The term “excimer” was first utilized as an abbreviation for “exciteddimer”, meaning two or more identical atoms comprising a molecule whichonly exists in an excited state, such as He₂ and Xe₂. Today, the term“excimer” has a broader meaning in the laser world and encompasses suchrare gas halides as XeCl (308 nm), KrF (248 nm), ArF (193 nm), KrCl (222nm), and XeF (351 nm). Several mercury-halides are also used as activegases in excimer lasers, such as HgBr. Even N₂, N₂ ⁺, CO₂ and F₂ (157nm) may be used as active media within excimer laser discharge chambers.As is apparent, many excimer lasers radiate at ultraviolet wavelengthsmaking them desirable for use as lithography tools. The KrF-excimerlaser emitting around 248 nm and the ArF-excimer laser emitting around193 nm are rapidly becoming the light sources of choice forphotolithographic processing of integrated circuit devices (IC's). TheF₂-laser is also being developed for such usage and emits light around157 nm.

[0007] To produce smaller feature sizes on IC chips, stepper and scannermachines are using expensive large scale submicron projection objectivesfor imaging a reticle onto a wafer surface with high diffracting-limitedprecision. The objectives operate at deep ultraviolet (DUV) wavelengths,such as the emission wavelengths of excimer lasers. For example, theKrF-excimer laser emitting around 248 nm is currently being used as aDUV radiation source. To reach greater resolution limits, the largefield objective lenses are designed and optimized in view of variouspossible and discovered imaging errors. The design optimization of theobjectives is, however, inadequate to meet the precision demands ofsub-quarter micron lithographic technology.

[0008] One way to improve the resolvability of structures on IC chips isto use more nearly monochromatic source radiation, i.e., radiationhaving a reduced bandwidth, Δλ. Other strategies include using shorterabsolute wavelength, λ, radiation such as that emitted around 193 nm and157 nm by ArF- and F₂-lasers, respectively, and increasing the numericalaperture (NA) of the projection lens.

[0009] The smallest structure resolvable on an IC chip depends on the“critical dimension” (CD) of the photolithography equipment being used:${{CD} = {K_{1} \cdot \frac{\lambda}{NA}}};{where}$

[0010] NA is a measure of the acceptance angle of the projection lens, λis the wavelength of the source radiation, and K₁ is a constant aroundapproximately 0.6-0.8. Simply increasing the numerical aperture NA toreduce the critical dimension CD simultaneously reduces the depth offocus DOF of the projection lens by the second power of NA:${{DOF} = {K_{2} \cdot \frac{\lambda}{({NA})^{2}}}};{where}$

[0011] K₂ is a constant around approximately 0.8-1.0. This complicateswafer adjustment and adds further strain on the demand for improvedchromatic correction of the projection lenses. Additionally, increasingthe numerical aperture NA to reduce the critical dimension CD forachieving smaller structures requires a decrease in the bandwidth Δλ oflaser emission according to:${{\Delta\lambda} = {K_{3} \cdot \frac{\lambda}{({NA})^{2}}}};{where}$

[0012] K₃ is a constant dependent on parameters associated with theprojection lens(es). Each of the above assumes that such other laserparameters as repetition rate, stability, and output power remainconstant.

[0013] Some techniques are known for selecting and for narrowing laseremission bandwidths including using optically dispersive elements suchas etalons, gratings and prisms, as well as modified resonatorarrangements. See U.S. Pat. No. 5,095,492 to Sandstrom (disclosing adispersive grating having a concave radius of curvature); U.S. Pat. No.5,559,816 to Basting et al. (disclosing a technique using thepolarization properties of light); U.S. Pat. No. 5,150,370 to Furuya etal. (disclosing a fabry-perot etalon within the laser resonator); U.S.Pat. Nos. 5,404,366, 5,596,596 and E.U. Patent Pub. No. 0 472 727, eachto Wakabayashi et al. (disclosing a concave outcoupler and a fixedaperture within the laser resonator); U.S. Pat. No. 4,829,536 toKajiyama et al. (disclosing angularly offset etalons).

[0014] Using this available knowledge, the bandwidth of laser emission,e.g., which is naturally around 500 pm for a KrF-excimer laser, can bereduced to Δλ≈0.8 pm, sufficient to meet the demands of currentprojection lenses (NA≈0.53) for producing quarter micron shipstructures. Further improvements in projection objectives (NA ≈0.8)combined with a further reduction in laser emission bandwidths(Δλ≈0.4-0.6 pm) are expected to reduce the critical dimension CD usingKrF-excimer laser sources down to CD≈0.18 microns. See J. Mulkens etal., Step and Scan Technology for the 193 nm Era, Third InternationalSymposium on 193 nm Lithography, Onuma, Japan (Jun. 29-Jul. 2, 1997).

[0015] The drawback to this significant bandwidth and CD reduction is acorrespondingly significant reduction in available laser output power.Narrow band efficiencies of twenty to forty percent of broadband outputpower are typical. There is thus a need for efficient spectral narrowingmethods which minimize power loss.

[0016]FIG. 1 shows a conventional excimer laser arrangement. A lasertube 1 contains a laser active medium (not shown) for emitting acharacteristic wavelength upon excitation pumping of the laser activemedium. A wavelength selection and narrowing assembly 2 includes adispersive grating 3 and at least one expanding and/or dispersive prism4. The grating 3 also serves to reflect substantially all of the laserlight incident upon it at a wavelength dependent angle. A narrow band ofthe light dispersed once through the prism 4 and incident upon thegrating 3 is reflected off of the grating 3 and back along the opticalpath of the arrangement, while all other wavelengths are reflected awayfrom the optical path. The arrangement is completed with an outputcoupling mirror 5 which reflects a portion of the resonating band andallows the rest to continue unreflected ultimately defining the outputbeam of the system.

[0017] The excimer laser arrangement of FIG. 2 includes all of theelements of FIG. 1 except the output coupling mirror 5, and furtherincludes a beam splitter 6 and a highly reflective mirror 8. The beamsplitter 6 serves as an output coupler reflecting the narrow band laseremission 9 from the optical path of the resonating beam. A highlyreflective mirror 8 is used instead of the partially reflecting outputcoupling mirror 5 of the arrangement of FIG. 1.

SUMMARY OF THE INVENTION

[0018] The present invention sets forth an apparatus and method forbandwidth narrowing of an excimer laser to Δλ≈0.6 pm or less with highspectral purity and minimized output power loss. Additional and/ormodified optical elements within the laser resonator are used. Outputstability with respect to pulse energy, beam pointing, beam size andbeam output location are also improvements of the present invention.

[0019] An apparatus according to the present invention is an excimerlaser including an active laser medium for generating a broadbandspectral beam at an original wavelength, means for narrowing thewavelength and/or selecting a spectral line of the generated broadbandspectral beam, a resonator and a means for outcoupling the resonatingband. Means for adapting or matching the divergence of the intracavityrays is further included in the apparatus according to the presentinvention for optimizing the combination of output power, spectralpurity and bandwidth of the output beam of the excimer laser. A methodaccording to the present invention includes selecting and aligning thedivergence adapting or matching means such that the combination ofoutput power, spectral purity and bandwidth of the output beam of theexcimer laser is optimized.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020]FIG. 1 shows a conventional excimer laser arrangement includingoutput coupling via a partially reflective resonator mirror.

[0021]FIG. 2 shows a conventional excimer laser arrangement including aphase retardation prism and output coupling via a beam splitter.

[0022]FIG. 3 shows an excimer laser arrangement according to a firstembodiment of the present invention.

[0023]FIG. 4 shows an excimer laser arrangement according to a secondembodiment of the present invention.

[0024]FIG. 5A shows an excimer laser arrangement according to a thirdembodiment of the present invention.

[0025]FIG. 5B shows an excimer laser arrangement according to a fourthembodiment of the present invention.

[0026]FIG. 6 shows an excimer laser arrangement according to a fifthembodiment of the present invention.

[0027]FIG. 7 shows a calculated output spectrum for the excimer laserarrangement of FIG. 1.

[0028]FIG. 8 shows a calculated output spectrum for the excimer laserarrangement of FIGS. 5A and 5B.

[0029]FIG. 9 shows a calculated output spectrum for an excimer laserarrangement wherein a cylindrical lens is placed between the laseractive medium and a beam expander.

[0030]FIG. 10 shows a calculated output spectrum for the excimer laserarrangement of FIG. 3.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

[0031] Spectral laser emissions propagate with a full-angle of beamdivergence after n round trips within the resonator of an excimer laserapproximately according to:${\theta_{0} \approx \frac{4a}{L \cdot (n)^{1/2}}};{where}$

[0032] a is the aperture radius (or its geometrical equivalentcorresponding to the geometry of the aperture), L is the length of theresonator, and n is the number of round trips an average photon emittedfrom the laser active medium traverses within the resonator beforeoutcoupling through, e.g., the outcoupling mirror 5 of the arrangementof FIG. 1, or the beam splitter 6A of the arrangement of FIG. 2. See S.Kawata, I. Hikima, Y. Ichihara, and S. Watanabe, Spatial Coherence ofKrF Excimer Lasers, Appl. Opt., vol. 31, page 387 (1992). Whendispersive elements are used for wavelength narrowing, the angulardivergence corresponds to a finite bandwidth:

θ₀≈Δλ₀.

[0033] Different parts of the lateral laser beam comprise differentspectral portions of the spectrally narrowed beam. See R. Sandstrom,Measurements of Beam Characteristics Relevant to DUV Microlithography ona KrF Excimer Laser, SPIE: Microlithography III, vol. 1264, 505, 511(1990) (showing in FIG. 8 the variation in spectral content of apreviously horizontally dispersed beam as a vertical slit mask selectsout portions of the beam, scanning horizontally from beam center to theright). The side wings, or outer wavelengths, of the spectral band ofthe excimer laser contain a significant amount of energy which cannot beignored due to its effect of diminishing the spectral purity and outputpower of the output beam. See Sandstrom (above), at 507-11. These sidewings also contribute to the width of the band.

[0034] Furthermore, spectral narrowing generally occurs in any laserresonator according to the spectral narrowing effect:${{\Delta\lambda}_{L} \approx \frac{{\Delta\lambda}_{0}}{(n)^{1/2}}};{where}$

[0035] n is the number of round trips, Δλ₀ is the bandwidth narrowed byoptical components within the resonator, and Δλ_(L) is the laseremission bandwidth. Effective spectral narrowing of a laser beamtypically requires a large number n of round trips. Since an excimerlaser usually has a few round trips, the natural spectral narrowingassociated with other types of lasers is not achieved. Further, in anexcimer laser, the side wings are amplified at the expense of the centerof the band, since they are within the divergence/acceptance angle ofthe beam during the few round trips traversed by the beam within theresonator.

[0036] At a given dispersive power, the degree of divergencecompensation has to be carefully adapted to the number of round trips,since they, together with the resulting bandwidth, spectral purity andoutput power of the emitted beam, are interdependent. Spectral purity isa measure of the spectral energy distribution within a narrow centralregion around the line center, e.g., within a 2 pm limit. The spectralpurity may also be defined as the energy within a specified wavelengthinterval divided by the total energy.

[0037]FIG. 3 shows an arrangement of a resonator of an excimer laser inaccordance with a first embodiment of the present invention. Thearrangement includes an active laser medium 1 for emitting light havinga characteristic wavelength. The arrangement further includes wavelengthselecting and narrowing optics 2 comprising a grating 3 and at least oneprism 4. The grating 3 serves as one of the two reflecting resonatorsurfaces of the first embodiment. The grating 3 reflects substantiallyall light incident upon it, each wavelength at a different angle. Theother reflecting surface is a convex-curved, preferably cylindrical,surface of an output coupling mirror 15. The output coupler 15preferably transmits a portion of the light incident upon it andreflects the rest. Alternatively, the outermost radial portion of theresonating band simply misses the output coupling mirror 15, which has asmaller radius than the resonating beam at that point. In either eventor in another conventional way, the portion of the resonating band thatexits the resonator along the predetermined optical path afterencountering the output coupling mirror 15 defines the emitted outputbeam 16 of the laser. The resonator may thus be operated as an unstableresonator, or alternatively, additional optics may be used to stabilizethe resonator.

[0038] As mentioned above, the lateral laser beam comprises a spectrumof wavelengths, ordered according to wavelength, as an effect oftraversing the wavelength selection and narrowing optics 2. The opticsof the arrangement of the first embodiment are aligned such that thecenter of the resonating band strikes approximately at the center of themirror 15. That center portion is reflected back along the optical pathof the resonator.

[0039] Other wavelengths above and below the center of the band, arereflected off the mirror 15 at angles away from the optical path whereinthese angles are enhanced due to the convex nature of the mirror 15.Wavelengths that are sufficiently removed from the center of the bandare reflected at such a high angle that they no longer are accepted byapertures within the resonator, such as those surrounding the laseractive medium 1. These wavelengths will not be part of any subsequentlyemitted output beam 16. Since a smaller geometrical region of theresonating beam will be accepted by natural apertures of the resonator,the resonating band of the first embodiment comprises a narrower rangeof wavelengths than a resonating band of a conventional resonator.

[0040] The resonator of the first embodiment is an unstable resonator ifa light ray initially propagating parallel to the optical axis of thelaser cavity could not be reflected back between the two mirroredsurfaces 3 and 15 indefinitely without escaping from between the mirrors3 and 15, other than by outcoupling. That is, the angle a ray of theresonating beam makes with the optical axis will increase with thenumber of round trips the ray makes within the resonator. If the grating3 is flat and no additional focusing optics are provided in thearrangement of the first embodiment, then the first embodiment willinclude an unstable resonator by virtue of the convex output coupler 15.

[0041] The angle of a light ray incident upon the output coupler 15 overwhich this light reflected from the outcoupling mirror 15 is dispersedout of the resonating beam is the acceptance angle of the beam. Thesmaller the radius of curvature of the convex outcoupling mirror 15, thesmaller the acceptance angle of the beam. Consequently, the smaller theradius of curvature of the outcoupling mirror 15, the narrower the bandof wavelengths that the ultimate emitted beam 16 will comprise. Theradius of curvature is preferably constant over the surface of themirror 15, but may change with distance from the center, or along adiameter. The focal length of the mirror 15 is preferably in the rangeof several meters, as are most curved components of the presentinvention.

[0042] The narrowing of the beam 16 is not achieved without a price.Generally, with all else being the same, the narrower the bandwidth ofthe beam, the weaker the output power of that beam 16. At some point,the beam 16 can be so narrowed that its output power is not sufficientto perform adequate lithography. However, the radius of curvature of theconvex mirror 15 can be selected appropriately, such that the outputpower of the beam 16 is sufficient, while the desired bandwidthnarrowing is achieved.

[0043] Thus, a first advantage of the first embodiment over aconventional arrangement having a, e.g., flat outcoupling mirror 5, suchas that of FIG. 1, is that the spectral purity of the beam 16 of thefirst embodiment is enhanced over the prior art. A second advantage isthat the bandwidth of the beam may be more greatly narrowed than a priorart beam while maintaining adequate output power, since the side wingsof the resonating beam are dispersed out by the outcoupling mirror 16and do not absorb power being amplified. Available power is thenfocusable on amplifying a more useful narrow central portion of theemission band 16. In a preferred embodiment, the radius of curvature ofthe outcoupling mirror 15 is adjustable to optimize the combination ofoutput power, bandwidth and spectral purity of the emitted beam 16.

[0044] The present invention is capable of improving spectral purity bybetween 20% and 50%, or more while power loss is kept to less than 10%.Power loss may be defined as the difference between the power of a stateof the art laser and the power of a laser according to the presentinvention, divided by the power of a state of the art laser. Typicalline narrowing efficiency is between 0.2 and 0.4, and particularly isaround 0.3 for lasers used in lithography.

[0045]FIG. 4 shows an arrangement of a resonator of an excimer laser inaccordance with a second embodiment of the present invention. The secondembodiment has an active laser medium 11, wavelength selection andnarrowing optics 12 including a grating 3, which preferably serves alsoas one of two reflecting surfaces of the resonator, and at least oneprism 14, and an output coupler 25. A highly reflective mirror mayalternatively perform the reflective function of the grating 3. Theoutput coupler 25 may be similar to the conventional output coupler 5 ofFIG. 1, or it may be similar to the output coupler 15 having areflecting surface with a convex radius of curvature of FIG. 3, or maybe another operable output coupler 25.

[0046] The laser active medium 11 is contained within a housing having afirst optical window 17A and a second optical window 17B to facilitateentrance and exit of the resonating beam. The windows 17A and 17B eachcomprise one or more conventionally UV transparent materials such ascrystalline quartz, CaF₂ and/or MgF₂, for example.

[0047] At least one surface of at least one, and preferably both,windows 17 a and 17B is curved. Both surfaces of one or both windows 17Aand 17B may be curved, but preferably only the outer surfaces are curvedas shown in FIG. 4. The effective total radius of curvature of allcurved surfaces of the windows 17A and 17B is selected to match or adaptthe divergence of the resonating beam and optimize the combination oflaser output power, bandwidth and spectral purity. By matching oradapting the divergence of the resonating beam, the angles of light raysof the resonating beam relative to the optical axis are changed tominimize the angle of the light rays relative to the optical axis. Thedivergence is matched or adapted in the present invention to optimizespectral purity and bandwidth. Divergence adapting is provided in thepresent invention by focusing elements and/or by the cutting of rayswith an angle relative to the optical axis greater than a specifiedangle by one or more apertures.

[0048] The prism 14 of the second embodiment has a first curved surface18A and a second curved surface 18B through which the resonating beamenters and exits the prism 14. Alternatively, only one surface 18A or18B may be curved. Another arrangement of the wavelength narrowing andselection optics 12 is possible wherein the resonating beam enters andexits the prism through the same curved surface. The curvature of eachsurface 18A and 18B is preferably convex. Alternatively, one may beconcave or the radius of curvature may change with position on one orboth surfaces 18A and 18B.

[0049] A first advantage of the second embodiment over a conventionalarrangement such as that shown in FIG. 1 is that the radius of curvatureof each of the surfaces 17A, 17B, 18A and 18B may be selected to matchthe divergence of the laser beam, and optimize output power, bandwidthand spectral purity. A second advantage is that the surfaces 18A and 18Bof the prism 14 and/or the surfaces 17A and 17B of the housingcontaining the active laser medium 11 may be used to expand or narrowthe resonating beam, depending on what is needed in the arrangementconsidering the properties and alignment of the other optics in thearrangement.

[0050]FIG. 5A shows an arrangement of a resonator of an excimer laser inaccordance with a third embodiment of the present invention. The thirdembodiment includes an active laser medium 21, wavelength selection andnarrowing optics 22 including a grating 3, which preferably serves alsoas one of two reflecting surfaces of the resonator, and at least oneprism 24, and an output coupler 35. A highly reflective mirror mayalternatively perform the reflective function of the grating 3. Theoutput coupler 35 may be similar to the conventional output coupler 5 ofFIG. 1, or it may be similar to the output coupler 15 having areflecting surface with a convex radius of curvature of FIG. 3, or maybe another operable output coupler 35. An output beam 16 is transmittedpast the output coupler 35. The prism 24 and housing of the active lasermedium 21 may be configured as in the either of the first or the secondembodiments of FIGS. 3 and 4, respectively, or otherwise conventionally.

[0051] The third embodiment also includes an aperture 19 located withinthe resonator arrangement of FIG. 5A. The aperture 19 is preferablylocated near the grating 3 as shown in FIG. 5A, but may be located atvarious locations along the optical path of the resonating beam. Morethan one aperture may be placed along the optical path of the resonatingband. The aperture 19 is preferably adjustable to optimize thecombination of the output power, the bandwidth and the spectral purityof the output beam 16. The aperture 19 is blocking highly divergentbeams, i.e., beams having a large angle relative to the optical axis ofthe resonator, primarily to improve spectral purity. When the aperture19 is located close to the grating 3, the output power is notsignificantly affected by the presence of the aperture 19. An advantageof the third embodiment is that the spectral purity, bandwidth andoutput power of the output beam 16 are optimized over those of aconventional arrangement such as that described in FIG. 1.

[0052]FIG. 5B shows an arrangement of a resonator of an excimer laser inaccordance with a fourth embodiment of the present invention. The fourthembodiment of FIG. 5B includes all of the elements of the thirdembodiment of FIG. 5A. Additionally, the fourth embodiment includes abeam splitter 6C after the output coupler 35 which transmits an outputbeam 26 and reflects a portion of the output of the output coupler 35.The reflected portion is received by a high resolution spectrometer 29for determining the wavelength and waveform characteristics of theoutput beam 26. A second beam splitter 6D is inserted to direct aportion of the output beam 26 toward an energy detector 28. The outputsof each of the detector 28 and the spectrometer 29 are received by acomputer 30 and processed. The computer then determines how the opticsof the arrangement should be modified to optimize the laser output 26with regard to the combination of output power, bandwidth and spectralpurity. The optics may then be manually or automatically adjusted inaccordance with the computer's instructions/suggestions. Particularlywith respect to the fourth embodiment, the aperture size may bemodified. Generally, the detector 28, the high resolution spectrometer29 and the computer 30 may be used with any of the embodiments of thepresent invention to help achieve the task of optimizing the combinationof the output power, the bandwidth and the spectral purity of the outputbeam, e.g., 26. Alternatively with respect to the fourth embodiment, afeed back circuit may be used for real time monitoring of the bandwidth,output power and spectral purity of the output beam 26 and adjustment ofthe aperture 19.

[0053]FIG. 6 shows an arrangement of a resonator of an excimer laser inaccordance with a fifth embodiment of the present invention. The fifthembodiment preferably includes wavelength selection and narrowing optics32 including the prism 4 of the first embodiment of FIG. 3, and thehousing for the laser active material 1 of the first embodiment of FIG.3. Alternatively, one or both of these elements 1, 4 may be substitutedby another element disclosed in one or more other embodiments of thepresent invention, e.g., the second embodiment. Additionally, the fifthembodiment includes a curved grating 13 and a curved output couplingmirror 45. The two curved optical surfaces together form an unstableresonator configuration. The curvature of each element 13, 45 may beconvex or concave, but preferably the output coupling mirror 45 isconvex like the output coupler 15 of the first embodiment and thegrating is concave, like the grating disclosed as element 40 in U.S.Pat. No. 5,095,492 to Sandstrom. Preferably, the combination of thecurvatures of the output coupler 45 and the grating 13 cause theresonator of the fifth embodiment to be unstable to match the divergencefor optimizing the combination of the output power, the bandwidth andthe spectral purity of the output beam 36. In addition, the radius ofcurvature of either the grating 13 or the output coupler 45, or both,may be adjustable. The resonator of the fifth embodiment may be, andpreferably is, an unstable resonator, such as that described withrespect to the first embodiment.

[0054]FIG. 7 shows a peak embodying the spectral distribution of theoutput beam 10 of FIG. 1. The output beam 10 is determined to have abandwidth of 1.1 pm, calculated as the full-width at half-maximum (FWHM)of the peak of FIG. 8 embodying the spectral distribution of the outputbeam 10.

[0055]FIG. 8 shows a peak embodying the spectral distribution of theoutput beam 16 of FIG. 5A, wherein the optical elements of thearrangement of the third embodiment included those included in thearrangement of FIG. 1 and an aperture 19 in front of the grating 3. Theaperture 19 used in obtaining the spectrum of FIG. 8 reduced thebandwidth from 1.1 to 0.5 pm by geometrically halving the divergentoutput beam in front of the grating 3.

[0056]FIG. 9 shows a peak embodying the spectral distribution of theoutput beam 46 of another arrangement. An additional optical element 27used to obtain the spectrum of FIG. 9 was a cylindrical lens, having afocal length of preferably several meters, placed between the housingfor the laser active material 21 and the prism 24. The prism 24 used wasa prism expander such as that described with respect to the secondembodiment of FIG. 4.

[0057]FIG. 10 shows a peak embodying the spectral distribution of theoutput beam 16 of the first embodiment of FIG. 3. The bandwidth wasreduced from 1.1 to 0.5 pm by using the convex-curved output coupler 15instead of the conventional output coupler 5 of FIG. 1.

[0058] An advantage of all of the above embodiments and improvements isthat the bandwidth of the output beam of the excimer laser system to beused in microlithographic applications is reduced, while the overalllaser efficiency is influenced only slightly. The reason is that onlythe central or principal part of the resonating beam traverses the mainamplification region of the laser active medium after it has encounteredone of the improved or additional optical elements of the presentinvention. Moreover, an improvement in spectral purity and stabilizationof the beam location, pointing and exit positions is observed when oneof the embodiments or improvements of the present invention is used overthat of a conventional arrangement such as that shown in FIG. 1. Thecombination of output power, bandwidth and spectral purity is optimizedby using or combining one or more embodiments of the present inventionby decreasing an acceptance angle of the resonating beam and/or matchingor adapting the divergence of the resonating beam.

What is claimed is:
 1. A method of controlling a spectral parameter ofan output beam from an excimer or molecular fluorine laser including alaser active medium, a resonator, an output coupler, a processor, anenergy detector, a spectrometer and a wavelength selection unit,comprising the steps of: operating the excimer or molecular fluorinelaser including an optical component having a curved surface within theresonator for providing the output beam with an improved spectralpurity; measuring a spectral parameter of the beam with a spectrometer,the spectral parameter being selected from the group consisting ofspectral purity and bandwidth; sending signals to the processor based onthe measuring of the spectral parameter; and adjusting the opticalcomponent within the resonator for controlling the spectral parameter ofthe beam based on the spectral parameter signals sent to the processor.2. A method of controlling a spectral parameter of an output beam froman excimer or molecular fluorine laser including a laser active medium,a resonator, an output coupler, one or more processors, an energydetector, a spectrometer and a wavelength selection unit, comprising thesteps of: operating the excimer or molecular fluorine laser including anoptical component having a curved surface within the resonator forproviding the output beam with improved spectral purity; measuring beamenergy and a spectral parameter with the energy detector andspectrometer, respectively, the spectral parameter being selected fromthe group consisting of spectral purity and bandwidth; sending signalsto the one or more processors indicative of the beam energy and spectralparameter; and adjusting the optical component within the resonator forcontrolling the spectral parameter of the beam based on the spectralparameter signals sent to at least one of the one or more processors. 3.The method of any of claims 1 or 2, wherein said adjusting of saidoptical component further for adapting a divergence of the output beam.4. The method of any of claims 1 or 2, wherein the optical componentincludes a refractive portion which refracts the beam.
 5. The method ofany of claim 1 or 2, further comprising the step of adjusting a geometryof an aperture for further improving the spectral purity of the beam. 6.The method of any of claims 1 or 2, wherein the inclusion of the opticalcomponent having the curved surface within the resonator and theperformance of the aligning step causes the spectral purity of theoutput beam to improve by between 20% and 50% and the output power toreduce by less than 10%.
 7. The method of any of claims 1 or 2, whereinthe adjusting step includes adjusting a curvature of said curved surfaceof the optical component.
 8. The method of any of claims 1 or 2, whereinthe optical component is a resonator reflector of said resonator.
 9. Themethod of claim 8, wherein the optical component is the output coupler.10. The method of any of claims 1 or 2, wherein the adjusting step isautomatically initiated by the processor when a spectral parametersignal is sent to the processor.
 11. The method of any of claims 1 or 2,wherein the adjusting step is manually performed.
 12. A method ofcontrolling a spectral parameter of an output beam from an excimer ormolecular fluorine laser including a laser active medium, a resonator,an output coupler, a processor, an energy detector, a spectrometer and awavelength selection unit, comprising the steps of: operating theexcimer or molecular fluorine laser including an optical componentwithin the resonator for providing the output beam with an improvedspectral purity; measuring a spectral parameter of the beam with aspectrometer, the spectral parameter being selected from the groupconsisting of spectral purity, wavelength and bandwidth; sending signalsto the processor based on the measuring of the spectral parameter; andadjusting the optical component within the resonator for controlling thespectral parameter of the beam based on the spectral parameter signalssent to the processor.
 13. A method of controlling a spectral parameterof an output beam from an excimer or molecular fluorine laser includinga laser active medium, a resonator, an output coupler, one or moreprocessors, an energy detector, a spectrometer and a wavelengthselection unit, comprising the steps of: operating the excimer ormolecular fluorine laser including a first optical component within theresonator for providing the output beam with improved spectral purity;measuring beam energy and a spectral parameter with the energy detectorand spectrometer, respectively, the spectral parameter being selectedfrom the group consisting of spectral purity, wavelength and bandwidth;sending signals to the one or more processors indicative of the beamenergy and spectral parameter; and adjusting the optical componentwithin the resonator for controlling the spectral parameter of the beambased on the spectral parameter signals sent to at least one of the oneor more processors.
 14. A method of controlling a spectral parameter ofan output beam from a molecular fluorine laser having a wavelengtharound 157 nm for use as source radiation for producing structures on ICchips, the molecular fluorine laser including a molecular fluorine laseractive medium, a resonator, an output coupler, a processor, an energydetector, a spectrometer and a wavelength selection unit, comprising thesteps of: operating the molecular fluorine laser including at least onewavelength selection optical component of said wavelength selection unitwithin the resonator for controlling a spectral parameter of the outputbeam having said wavelength around 157 nm, the spectral parameter beingselected from the group consisting of spectral purity and bandwidth;measuring the spectral parameter of the beam with the spectrometer;sending signals to the processor based on the measuring of the spectralparameter; and adjusting the at least one wavelength selection opticalcomponent of said wavelength selection unit within said resonator forcontrolling the spectral parameter of the output beam based on saidsignals sent to the processor.
 15. The method of claim 14, wherein saidat least one wavelength selection optical component includes a resonatorreflector.
 16. The method of claim 14, further comprising the step ofadjusting the beam energy of the output beam based on the beam energysignals sent to the processor.
 17. An excimer or molecular fluorinelaser, comprising an active laser medium for emitting an output beam; aresonator defining an optical path intersecting said active medium; atleast one line-narrowing optical component for narrowing the bandwidthof the output beam; an adjustable aperture within the resonator forcontrolling spectral purity of the output beam; an energy detector and aspectrometer each for receiving a portion of the spectral beam; and oneor more processors for receiving signals from each of the energydetector and the spectrometer, and wherein the adjustable aperture isconfigured to be adjustable based on an adjustment signal received fromat least one of the one or more processors, the adjustment signal beingdetermined based at least on spectral information received from thespectrometer.
 18. An excimer or molecular fluorine laser, comprising: anactive laser medium for generating a spectral beam at an originalcentral wavelength; a resonator including a first reflecting surface anda second reflecting surface, an optical path intersecting said activemedium being defined for said resonator for generating a laser beam, atleast one of said first and second reflecting surfaces being a curvedsurface including an adjustable curvature; a wavelength selector forselecting a wavelength band from the spectral beam including a beamexpander and a grating, the grating also serving as said firstreflecting surface; an energy detector and a spectrometer each forreceiving a portion of the spectral beam; one or more processors forreceiving signals from each of the energy detector and the spectrometer,and wherein the curvature of the curved surface is automaticallyadjusted when a signal is received from at least one of the one or moreprocessors based on information received from the spectrometer.
 19. Thelaser of claim 18, further comprising an aperture for adapting adivergence of the resonating beam to improve spectral purity of thelaser beam.
 20. An excimer or molecular fluorine laser, comprising anactive laser medium for emitting an output beam; a resonator defining anoptical path intersecting said active medium; at least oneline-narrowing optical component for narrowing the bandwidth of theoutput beam, including at least one adjustable optic for controlling thewavelength of the output beam; an energy detector and a spectrometereach for receiving a portion of the beam; and one or more processors forreceiving signals from each of the energy detector and the spectrometer,and wherein the adjustable optic is configured to be adjustable based onan adjustment signal received from at least one of the one or moreprocessors, the adjustment signal being determined based at least onspectral information received from the spectrometer.
 21. A molecularfluorine laser for generating a 157 nm laser beam for providing improvedresolvability of structures on IC chips as a lithographic processingtool, comprising: an molecular fluorine laser medium for emittingradiation at an original central wavelength around 157 nm; a resonatorincluding a first reflecting surface and a second reflecting surface, anoptical path intersecting said active medium being defined for saidresonator for generating the laser beam; a wavelength selector includingat least one wavelength selection optical element for selecting awavelength band from the spectral distribution of the emitted radiation;an energy detector and a spectrometer each for receiving a portion ofthe spectral beam; and one or more processors for receiving signals fromeach of the energy detector and the spectrometer, and wherein thewavelength selection optical component is automatically adjusted foradjusting a spectral parameter of the selected wavelength band when asignal is received from at least one of the one or more processors basedon information received from the spectrometer.
 22. The laser of claim21, wherein an adjustment of the beam energy of the output beam isautomatically initiated by the processor based on information receivedfrom the energy detector.